1. Field of the Disclosure
The present disclosure is related to methods for measuring a resistance of an earth formation downhole. In particular, the present disclosure is related to providing a correction for standoff in resistance measurements.
2. Description of the Related Art
In oil exploration, resistivity measurements of an earth formation are typically obtained by lowering a resistivity tool into a borehole penetrating the formation. Generally, the tool includes at least one current transmitter which introduces current into the formation and at least one return electrode at which current returns to the tool. Resistance is measured between the two electrodes. Ideally, current flows directly from the current transmitter through the earth formation and returns through the return electrode. In typical downhole situations, however, the resistivity tool is separated from the formation by a gap filled with oil-based mud (OBM) that is used for the purposes of drilling. The gap may be uniform along the length of the borehole or may experience variations due to borehole rugosity. The presence of mud and borehole rugosity affects the value of resistivity measurements obtained from the formation.
Several methods are used to reduce the effect of borehole irregularities and standoff, such as obtaining dual frequency measurements and post-processing based on a corresponding calculation; deriving a mud parameter α=(ρmω∈m∈0)−1 from direct mud cell measurements; and obtaining multi-standoff measurements and post-processing based on a corresponding calculation; among others. These methods rely on obtaining an additional measurement to derive the mud parameter and can require new tool designs and/or assumptions about the dispersive characteristics of the mud. Thus, there is a need to provide a correction to resistivity measurements in the presence of the oil-based mud in a gap based on an analysis of the data and which does not require any additional measurements or prior information about the mud's electrical properties. The present disclosure satisfies this need.